1. Field of the Invention
This invention relates to a melt spinning process for forming fibers and fibrous webs. In particular, very fine fibers can be made and collected into a fibrous web useful for selective barrier end uses such as in the fields of air and liquid filtration, flame retardancy, biomedical, battery and capacitor separators, biofuel membranes, cosmetic facial masks, biomedical applications, such as, hemostasis, wound dressings and healing, vascular grafts, tissue scaffolds, synthetic ECM (extra cellular matrix), and sensing applications, electronic/optical textiles, EMI Shielding, and antichembio protective coatings.
2. Background
Centrifugal atomization processes are known in the art for making metal, metal alloy and ceramics powers. Centrifugal spinning processes are known in the art for making polymer fibers, carbon pitch fibers and glass fibers, such as disclosed in U.S. Pat. Nos. 3,097,085 and 2,587,710. In such processes, the centrifugal force supplied by a rotational disc or distribution disc produces enough shear to cause the material to become atomized or to form fibers. However, centrifugal spinning has only been successfully used for the production of fibers with diameters larger than micron size.
There is a growing need for very fine fibers and fibrous webs made from very fine fibers. These types of webs are useful for selective barrier end uses. Presently very fine fibers are made from melt spun “islands in the sea” cross section fibers, split films, some meltblown processes, and electrospinning. However, these processes are generally limited to making non-commercial quantities of nanofibers because of their very low throughput.
In order to successfully produce nano or sub-micrometer diameter fibers by melt-spinning, drawing of the polymer must occur as a result of either mechanical or electrostatic forces acting on the melt jet at the spinneret. However, it is very difficult to generate the forces needed to create a reduction in diameter to the nanometer level. Melt-electrospinning has been conducted in some universities since the later 1970s, but there was no success reported in making nanofibers, especially from polyolefins, such as polyethylene (E) and polypropylene (PP).
Electrospinning and electroblowing are processes for forming fibers with sub-micron scale diameters from polymer solutions through the action of electrostatic forces and/or shear force. The fibers collected as non-woven mats have some useful properties such as high surface area-to-mass ratio, and thus have great potential in filtration, biomedical applications (such as, wound dressings, vascular grafts, tissue scaffolds), and sensing applications.
However, a vast majority of nanofibrous structures are produced by solvent based spinning processes. Chemicals used as solvents to dissolve many of the polymers being spun may leave residuals that are not compatible within the industry. With the intent of cleaner processing, environmental safety, and productivity, there is a persistent desire to produce fibers by melt spinning.
Spinning nanofibers directly from polymer melts would offer several advantages over solution based spinning: elimination of solvents and their concomitant recycling requirements, higher throughput, and spinning of polymers with low solvent solubility. Likewise, multi-component systems such as blends and composites would be more readily melt spun, because in many cases there is no common solvent for such blends. Finally, productivity would increase 10-500 fold and costs would drop significantly due to elimination of solvent recovery.
What is needed is a high throughput, cost effective and energy efficient process to melt spin nanofiber fibers and uniform fibrous webs.